This application claims the benefit of Korean Patent Application No. 2001-35809, filed on Jun. 22, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a driving circuit for an active matrix organic light emitting diode. Particularly, the present invention relates to a driving circuit for an active matrix organic light emitting diode capable of driving an organic light emitting diode (LED) by supplying unique power voltages to organic LEDs capable of emitting red, green and blue colors.
2. Discussion of the Related Art
LEDs are devices that emit light when electrons and holes recombine within P-N junctions of semiconductor diodes. In thermal equilibrium, electrons and holes do not recombine due to the presence of a band gap energy between energy levels of the electrons and the holes. However, when a forward bias voltage is applied to the P-N junction, electrons migrate from a P region to an N region and holes migrate from the N regions to the P region. Accordingly, the migrating electrons and holes recombine to thereby emit light.
LEDs are fabricated from group III-V, II-VI, or Vxe2x80x94V semiconductor materials. The color of light emitted by the LEDs depends on the band gap energy of the P-N junction. The band gap energy may be controlled by the composition ratio of the aforementioned semiconductor materials.
Contrary to thin film transistor liquid crystal diodes (TFT-LCD), organic LED devices may be manufactured to emit red, green, and blue colors without the use of color filters. Instead, red, green and blue light may be emitted using various organic substances. Further the brightness of the light emitted by an organic LED depends on the voltage that is applied to it. Accordingly, an image may be displayed by organic LED devices without the use of a back light unit or color filters.
As mentioned above, organic substances capable of displaying red, green and blue light have voltage dependent display characteristics. The recombination efficiency and the brightness of different organic LEDs is different for any given voltage applied thereto.
FIG. 1 illustrates a block diagram of a related art data driver IC of a driving circuit used in an active matrix organic LED.
Referring to FIG. 1, the driving circuit of the active matrix organic LED device includes a gamma voltage generation unit 1 for generating gamma reference voltages (GMA1xcx9cGMA10) that are necessary for controlling brightnesses of organic LEDs capable of emitting red, blue, and green light; and a driving unit 2 for displaying an image upon receipt of a power voltage (VDD), a common ground voltage (GND) from a power supply unit (not shown), and the gamma reference voltages (GMA1xcx9cGMA10). The driving unit 2 also applies a current to the organic LED according to the corresponding gamma reference voltages (GMA1xcx9cGMA10) as determined by the data signal to the organic LED.
In the driving circuit of FIG. 1, identical gamma reference voltages are generated by the gamma reference voltage generation unit 1. Accordingly, the identical gamma reference voltages applied to respective organic LEDs to display red, green and blue colors.
The organic substances emitting the red, green and blue colors, however, do not have identical voltage dependent brightness characteristics. Therefore, applied voltage values corresponding to maximum brightness emissions by red, green, and blue LEDs are different.
Accordingly, when the common gamma reference voltages (GMA1xcx9cGMA10) generated by the gamma reference voltage generation unit 1 of FIG. 1 are applied, optimized brightness characteristics for each of the red, green, and blue organic LEDs in the active matrix organic LED device cannot be obtained.
FIG. 2 illustrates a detailed view of the driving unit shown in FIG. 1.
As shown in FIG. 2, the driving unit 2 includes an address shift register 10 for starting a driving operation by receiving a control signal, e.g., a clock signal (CLK), from a control unit (not shown); an input register 20 for receiving and storing the control signal from the address shift register 10 in addition to image data, e.g., RGB data from the control unit; a storage register 30 for storing, ordering according to respective addresses, and outputting the image data and control signal; a digital/analog converter 40 for receiving the ordered image data and control signal from the storage register 30, outputting analog image data, receiving the common power voltage (VDD) from the power supply unit and the plurality of gamma reference voltages (GMA1xcx9cGMA10) from the gamma voltage generation unit 1; and an output voltage driving unit 50 for receiving the analog image data and outputting the driving voltage.
Hereinafter, an operation of the driving unit 2 illustrated in FIGS. 1 and 2 will be described in detail.
When the control signal is inputted from the control unit to the address shift register 10, an enable signal corresponding to an address and comprising m number of bits is outputted on the basis of the control signal.
When given the m-bit enable signal, the input register 20 also receives i-bit image data comprising digital signals of RGB data from the control unit.
The input register 20 includes a storage means for displaying one frame of an image and has ixc3x97mxc3x973 bits of storage space to store the RGB data, m-bit enable signal, and i-bit image data.
When a next clock signal CLK is inputted to the input register 20, the stored data are initialized and moved to the storage register 30 and data for the next frame is stored therein. The storage register 30 has an identical size as the input register 20.
Next, the storage register 30 outputs i-bit image data, corresponding to the respective addresses.
The i-bit image data of the storage register 30 is outputted and converted into an analog video signal using an digital/analog conversion unit 40. The digital/analog conversion unit 40 receives the common power voltage (VDD) regardless of the RGB data and common gamma reference voltages (GMA1xcx9cGMA10).
The voltage value of the analog image signal, determined by the gamma reference voltages and power voltages, is the same regardless of the red, green and blue devices receiving the analog image signal. Accordingly, the organic LED devices, capable of emitting red, green, and blue light, present within the active matrix organic LED device cannot be driven to emit light having a preferred brightness.
The output voltage driving unit 50 applies the analog image signal to data lines of the respective pixels through common buffering techniques.
Using the driving circuit illustrated in FIGS. 1 and 2 to drive the active matrix organic LED, identical power and gamma reference voltages are applied to all of the organic LEDs, regardless of the colors they emit. Therefore, optimal brightness characteristics of the active matrix organic LED may not be realized.
Accordingly, the present invention is directed to a driving circuit for an active matrix organic light emitting diode that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a driving circuit for an active matrix organic LED, capable of applying different voltages that are appropriate for the brightness of the color of light to be emitted.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. Other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a driving circuit for an active matrix organic light emitting diode (LED) includes a gamma voltage generation unit for generating gamma reference voltages having specific values that correspond to organic LEDs capable of emitting different colors; and a driving unit for outputting a video data signal of a frame by receiving the gamma reference voltages having specific values and power voltages having specific values that correspond to organic LEDs capable of emitting different colors.
Additionally, a driving unit includes an address shift register for starting a driving operation by receiving the above control signal and outputting an enable signal; an input register for receiving, storing, and outputting the enable signal from the address shift register and the image data from a control unit; a storage register for receiving, storing, and outputting the image data and enable signal inputted by the input register; a digital/analog converter for outputting analog image data according to respective addresses by receiving image data from the storage register, the power voltage and common ground voltage from the power supply unit and the gamma reference voltages from the gamma voltage generation unit; and an output voltage driving unit for receiving the analog image data and outputting the data through data lines of pixels.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.